1. Field of the Invention
The present invention relates to a secondary battery and a method of manufacturing same, and more particularly, to a secondary battery and a method of manufacturing same in which the reliability of a safety device connected to a lead plate is improved.
2. Description of the Related Art
Representative examples of secondary batteries, which are rechargeable and can be manufactured to be small and to have a large capacitance, include nickel-hydrogen (Ni-MH) batteries, lithium batteries, and lithium ion batteries.
Such a secondary battery is manufactured by placing an electrode assembly, which is comprised of a positive electrode plate, a negative electrode plate, and a separator, in a battery case (can) made of aluminum or an aluminum alloy, injecting an electrolyte into the can, and sealing the can. The can made of aluminum or an aluminum alloy, which are lighter than other conductive metals, such as iron, is conducive to the manufacture of lightweight batteries and is resistant to corrosion even when used in a high voltage condition for a long period of time. The secondary battery includes an electrode port formed on the top of the can such that the electrode port is insulated from the can. This electrode port forms a positive electrode or a negative electrode of the battery. The battery can forms the negative electrode of the battery when the electrode port forms the positive electrode and forms the positive electrode when the electrode port forms the negative electrode.
The secondary battery sealed in the can is connected to a safety device, for example, a positive temperature coefficient (PTC) element, such as a positive thermistor, a thermal fuse, a protecting circuit module (PCM), and the like and is then arranged in a battery pack. Such a safety device is connected between the positive electrode and the negative electrode and cuts off a flow of current when the temperature of the battery rises too high or when the voltage of the battery suddenly rises due to overcharging or over-discharging, thereby preventing destruction of the battery.
The safety device is connected between the positive electrode and the negative electrode of the battery by a lead plate. The lead plate is made of nickel, a nickel alloy, or a nickel-plated stainless steel and has a predetermined hardness and conductivity.
U.S. Pat. No. 5,976,729 discloses a cell that improves reliability with a protecting circuit. In this patent, a lead plate made of nickel is previously welded to an external bottom surface of an aluminum can by laser irradiation. Another lead plate is resistance welded to the lead plate made of nickel and is connected to a safety device, such as a PCM, so that the safety device can be protected from laser irradiation.
However, the aluminum can is very thin, so that the intensity of a laser beam applied when welding the lead plate to the bottom surface of the can must be carefully controlled so as to prevent an electrolyte from leaking. In addition, sequentially connecting two lead plates to the bottom surface of the cell lowers working efficiency.